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338           ON  THE  CHANGE  OF REFBANGIBILITY  OF  LIGHT.
and yellow. This band fades away gradually at its less refrangible limit, where it is separated by a dark interval from the narrow well-defined red band of still lower refrangibility due to chlorophyll. At its more refrangible limit, however, it breaks off with unusual abruptness.
122.    When the light transmitted through such a sea-weed is subjected to prismatic analysis, in addition to one at least of the absorption bands due to chlorophyll, there is seen a band obliterating the yellow, another dividing the green from the blue, and a third, far less conspicuous, dividing the green into two. The whole of the green is absorbed more rapidly than the blue beyond, and not merely than the red, which last is the final tint.
123.    The red colouring matter is easily extracted by cold water from certain kinds of red sea-weed, if fresh gathered; but when once the plant has been.dried, the colouring matter cannot be  extracted in  any way that  I  know  of.     It  is  apparently insoluble  in  alcohol  and ether, and is decomposed by boiling. Cold water extracts only a trace of it after a long time.
124.    A piece of recently gathered red sea-weed, on being mashed with   cold   water,  readily gave   out   its   red   colouring matter.    When the residue was treated with alcohol, the fluid was almost immediately coloured green by chlorophyll, whereas this substance is only very slowly and  sparingly extracted by alcohol from dried sea-weeds.    A dried sea-weed may apparently be assimilated to an intimate mixture of gum and resin, which it would be very difficult to dissolve, whether it were attacked by water or alcohol.
125.    The solution of the red colouring matter was highly sensitive, exhibiting a copious dispersive reflexion of a yellowish orange light.    The transmitted light was pink or red, according to the thickness through  which   the  light passed.    When  this light was analysed, the same three absorption bands which have been already mentioned   were  perceived.    The   analysis   of the light transmitted by the fronds  of various  red  sea-weeds  had rendered it extremely probable that the  faint  division   in   the green did belong to the red colouring  matter;   but till  I  had obtained this matter in solution I did not feel certain  that it